The Ernie2 phenotype was identified among G3 mice of the pedigree R0801, some of which showed weight loss at day 10 after treatment with dextran sulfate sodium (DSS)-containing water, indicating susceptibility to DSS-induced colitis (Figure 1).

Nature of Mutation

Figure 2.Linkage mapping of the DSS-induced weight loss at day 10 after DSS treatment using a semidominant model of inheritance. Manhattan plot shows -log10 P values (Y-axis) plotted against the chromosome positions of 44 mutations (X-axis) identified in the G1 male of pedigree R0801. Normalized phenotype data are shown for single locus linkage analysis without consideration of G2 dam identity. Horizontal pink and red lines represent thresholds of P = 0.05, and the threshold for P = 0.05 after applying Bonferroni correction, respectively.

Whole exome HiSeq sequencing of the G1 grandsire identified 44 mutations. The increased susceptibility to DSS-induced colitis was linked by continuous variable mapping to a mutation in Ern2: a T to C transition at base pair 122,180,862 (v38) on chromosome 7, or base pair 5,365 in the GenBank genomic region NC_000073 within the donor splice site of intron 7. Linkage was found with an additive model of inheritance (P = 1.30 x 10-4) wherein two variant homozygotes departed phenotypically from seven homozygous reference mice and 11 heterozygous mice (Figure 2). The effect of the mutation at the cDNA and protein level have not examined, but the mutation is predicted to not result in disruption of the splice donor site; native splicing sites might be used. In the event that the mutation affects splicing, the most likely aberrant splicing result will be skipping of the 102-base pair exon 7 (out of 22 total exons (shown below), resulting in an in-frame deletion of 34 amino acids beginning after amino acid 161 of the protein.

IRE1β is expressed in the gastrointestinal epithelial cells and mucus cells in bronchial epithelia. During the unfolded protein response (UPR), both IRE1α and IRE1β can catalyze the processing of Xbp1 (4-6), although IRE1β is less efficient than IRE1α (1). IRE1α and IRE1β both activate c-Jun NH2-terminal kinase (JNK) in response to ER stress (7). IRE1β is unique from IRE1α in that IRE1β directly interacts with unfolded proteins rather than with GRP78 during conditions of ER stress (8).

IRE1β posttranscriptionally degrades Mtp(9), which is induced during enterocyte differentiation. Mtp encodes an ER chaperone required for lipid mobilization via apolipoprotein B (apoB) lipoproteins (9). MTP and apoB are also required for chylomicron assembly in the ER of enterocytes (10-12). Chylomicrons are vesicles synthesized by the intestine and function to transport large quantities of dietary fat and fat-soluble vitamins (12-14).

Ern2 knockout (Ern2-/-) mice are viable and healthy. However, when challenged with DSS, the Ern2-/- mice (in both the 129svev and C57BL/6 genetic backgrounds) developed colitis 3-5 days earlier than wild-type and heterozygous mice. In addition, the Ern2-/- mice exhibited mortality approximately 5 days earlier than wild-type or heterozygous mice. Further studies determined that loss of IRE1β expression promotes ER stress specifically in immature goblet cells (15). Examination of goblet cells from Ern2-/- mice determined that the ER was distended in the cells. Within the distended ER of the immature goblet cells, aberrant mucin accumulation was observed. Tsuru et al. proposed that the susceptibility to colitis may indicate a qualitative defect in mucins secreted by the goblet cells in the Ern2-/- mice (15). Similar to the Ern2-/- mice, the Ernie2 mice exhibit susceptibility to DSS-induced colitis indicating loss of function in IRE1β.